Ion implantation is a technique for introducing conductivity-altering impurities into semiconductor workpieces. During ion implantation, a desired impurity material is ionized in an ion source chamber, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is focused and directed toward the surface of a workpiece positioned in a process chamber. The energetic ions in the ion beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the material to form a region of desired conductivity.
Semiconductor workpieces are highly susceptible to particulate contamination that can detrimentally alter the conductivity characteristics of a workpiece. In order to mitigate such contamination, ion implantation is typically performed in high vacuum pressure environments. It is therefore necessary to employ transfer devices for moving workpieces into and out of such vacuum environments in an expeditious manner while simultaneously minimizing the creation and transmission of particulate matter.
Traditional batch-transfer load-lock systems for moving semiconductor workpieces into and out of vacuum environments typically include one or more load-locks that are each configured to receive one or more workpieces at a time. In some embodiments, a cassette of vertically-stacked semiconductor workpieces is transferred into a load lock from a side of the load-lock that is exposed to an atmospheric pressure environment. After the stack of workpieces has been loaded into a load-lock from the atmospheric side, the load-lock is sealed and an interior of the load-lock is pumped down to vacuum pressure. A side of the load-lock that is exposed to a high vacuum pressure processing environment is then opened and one or more of the workpieces in the stack are collected for subsequent transfer to an ion implanter. After the workpieces have been implanted, the above-described transfer process is performed in reverse to move the workpieces back to the atmospheric pressure environment where they may be collected for further processing.
A problem that is associated with traditional batch-transfer load-lock systems of the type described above is that semiconductor workpieces may be exposed to different amounts of particulate depending on their respective positions within a stack. For example, a workpiece that is at a top position in a stack may gather a greater amount of particulate than workpieces at lower positions within the stack that are shielded by workpieces at higher positions. This may result in uneven particulate contamination between workpieces, and therefore inconsistent conductivity characteristics, within a batch of semiconductor workpieces handled by a particular load-lock.